Monitoring and managing light leaf spot
2 Nov 2018
Light leaf spot (LLS) was often seen as a disease confined to northern areas, but it is now found across the length and breadth of the UK. Currently, the AHDB forecast is predicting a lower than usual risk on a national level but it’s an important disease to monitor and manage.
In this blog, Philip Walker, Plant Pathologist with ADAS, talks about managing LLS in winter oilseed rape (WOSR) crops this autumn.
The primary source of infection for LLS are ascospores (airborne spores) released from crop debris remaining in the field after harvest. Leaving this debris on the soil surface will increase the risk for neighbouring crops.
The AHDB LLS forecast is predicting a lower than usual risk, particularly in the south east. Despite the lower risk predicted nationally, the history of LLS on farm, the variety resistance rating and also the drilling date of the crop should be taken into account when planning a fungicide strategy.
In the field, the early symptoms of LLS are easily missed. Early LLS symptoms can sometimes resemble frost and scorch damage. Taking plants, placing them in a polythene bag and leaving in a warm room for 2 to 3 days will encourage symptoms to appear. This can be particularly useful where LLS is suspected, but key diagnostic characteristics e.g. the tiny white droplets around the lesion, are not visible.
Where LLS is known to be an issue, using more resistant varieties will help with disease management. Many growers are moving to using varieties with a better resistance rating where LLS has been a problem and there is now a minimum standard on the AHDB Recommended List of 6. Using more resistant varieties can give you more flexibility with spray timing as the disease will cycle more slowly. This can act as an insurance if conditions are not ideal for spraying when infection is first detected.
Autumn fungicide application
First phoma sprays, often applied in early to mid-October in response to the 10 to 20% threshold for treatment, are usually too early to have an effect on LLS. Applications in late October, November and December tend to give better control, however, it is not always possible to travel later in the year. It is difficult to predict how severe LLS will be in an individual crop during the season and an autumn fungicide will give early protection. This should be followed up by monitoring from January onwards, so a further fungicide application can be planned when symptoms are first seen.
LLS can be a difficult disease to control so an integrated approach, using a combination of variety resistance and chemical control, is best.
Strains with decreased sensitivity to azole fungicides have been found in the UK, however, the presence of these strains does not appear to be having an impact on field performance of azole chemistry at present.
We need to use fungicides wisely, implementing fungicide resistance management strategies, to prevent the selection for strains with the potential to be more problematic to growers in future.
Best practice starts by thinking about the fungicide programme as a whole and using effective co-formulations and tank mixes (single applications of two modes of action that are both effective against LLS) and alternating different modes of action throughout the whole programme. We are limited to azole and azole co-formulations to control LLS in the autumn, so opportunities to alternate azoles with non-azole options and use azoles in mixtures for other parts of the fungicide programme, should be considered.